This invention relates to a method and device for continuously annealing metallic ribbons. The invention also relates to magnetomechanical markers for electronic article surveillance and a method and an apparatus for making the same.
Amorphous ferromagnetic metals are typically produced by rapid solidification from the melt as a continuous, typically 20-30 xcexcm thickness ribbon. Due to their atomic structure they exhibit good soft magnetic properties in the as cast state. However, as for any magnetic material, their magnetic properties can be significantly enhanced by a subsequent heat treatment at elevated temperatures (annealing). In this way their properties can be precisely adjusted to the needs of a large variety of applications. Another purpose of the annealing treatment may be to give the ribbon a desired geometrical shape. Typically, when heat-treated at high enough temperatures the metal ribbon takes the geometrical shape it was subjected to during the heat treatment.
Among many applications (for example, in soft magnetic cores), amorphous ferromagnetic metals are widely used as a marker for electronic article surveillance (EAS). Such a marker typically is made of an elongated strip of an amorphous ribbon with well-defined, highly consistent soft magnetic properties. The latter provide the marker with signal identity in order to distinguish it from other objects passing through the interrogation zone of such a surveillance system.
Apart from well-defined magnetic characteristics, many sensor applications, such as markers for EAS, moreover need a substantially flat strip, or a strip with a small well-defined curvature. This is for example necessary to fit the sensor strip into a cavity without bending it. In particular for magnetoelastic sensors, such as acousto-magnetic EAS markers, such bending would result in a severe degradation of the magnetic performance due to magnetostrictive coupling.
One problem with amorphous ribbons is that they reveal a production-inherent longitudinal and/or transverse curvature (c.f. F. Varret, G. Le Gal and M. Henry in Journal of Material Science Vol. 24 (1989) pp. 3399-3402). The height of this curvature may range up to 1000 xcexcm and more (see below for definition of longitudinal curvature) and originates from thermally induced mechanical stresses during rapid solidification. The height of the curvature is extremely sensitive to the casting conditions, and in practice cannot be controlled in a reliable way. The annealing treatment must therefore also remove this initial curvature of the ribbon and give it a flat shape or a small pre-defined curvature.
A common way of performing the heat treatment is continuous annealing of the metal ribbon. That is the ribbon is fed from a supply reel located on one side of an oven, continuously transported through a zone of elevated temperatures in the oven, and then taken up on a take-up reel on the other side of the oven. In such a process the ribbon is given characteristic properties by careful choice of the annealing parameters such as the temperature profile in the oven and the duration of annealing, which is dependent upon the speed of the ribbon through the oven. A tensile stress, a magnetic field or an electric current applied during annealing can be further used to tailor the magnetic properties.
One way of heating the ribbon is wrapping it around a heated wheel as described in U.S. Pat. No. 5,684,459. In this way an initial longitudinal curvature of the ribbon can be removed within annealing times of a few seconds by bending the ribbon xe2x80x9cbackwardsxe2x80x9d against its initial curvature. However, this curvature-removal by counter-bending the ribbon is extremely sensitive to the annealing conditions. The curvature disappears only for a precise annealing time, dependent upon the initial curvature of the ribbon. If, for example, the ribbon is annealed for too long a time, it develops a strong curvature opposite to its original direction. Moreover the curvature reduction affects the magnetic properties. Thus, one has to accept a compromise between curvature reduction and magnetic characteristics.
Another common method is to transport the ribbon in a straight way through an oven such as for example described in U.S. Pat. Nos. 5,757,272, 5,676,767, 5,786,762 and 6,011,475. In this method, the ribbon is guided through the channel of an annealing fixture, which acts as a heat reservoir and which supports the ribbon, such that its straightness during annealing is maintained. Since the ribbon is kept straight, any longitudinal curvature is removed provided the ribbon is exposed to a certain minimum annealing temperature and a certain minimum annealing time. Alternatively, the cross-section of the annealing fixture may have a curved profile in order to give the ribbon a small transverse curl, which enhances the longitudinal bending stiffness and, thus, reduces any longitudinal curvature. The longitudinal curvature-removal process is then largely independent of the precise annealing conditions. Accordingly, the annealing parameters necessary for the magnetic characteristics can thus be optimized independently and without compromise.
However, the major problem of the just mentioned process is associated with the annealing speed. For reasons of process efficiency it is highly desirable to have as high an annealing speed as possible. Yet, in practice, if the annealing speed exceeds a certain limit (for a 2 m long oven typically in the range from 10 to 20 m/min) the desired properties (such as the magnetic characteristics or the flatness) degrade rapidly with increasing speed. Trivially, the annealing speed can always be increased by constructing a correspondingly longer oven. Yet the latter solution significantly increases the cost of the annealing equipment and, thus, again reduces process efficiency.
According to the state of the prior art for continuous annealing the process efficiency is limited in terms of a maximum annealing speed above which the achievable properties degrade. The inventors have recognized that this problem is not necessarily related to the short annealing times by itself, which are associated with high speeds, but rather is a question of the heat transfer into the ribbon. It is known that a good and quick heat transfer requires direct contact of the metallic ribbon with a heat reservoir, which has a good thermal conductivity. This is for example the case for direct metal-metal contact. Thus, for example, wrapping the ribbon around a heated metallic roller provides an excellent heat transfer into the ribbon and allows high annealing speed. However, the disadvantage is that the ribbon takes the curvature of the heated roller or one has to accept a compromise between this curvature and the magnetic characteristics. Annealing the ribbon in a straight oven resolves this deficiency but only with a significantly reduced annealing speed. The reason is that the heat transfer into the ribbon occurs via the gas atmosphere in the oven, which is a relatively slow process. As a consequence, if the annealing speed becomes too fast, the material does not heat up sufficiently and the achievable properties (such as the magnetic characteristics or the flatness) degrade rapidly with increasing annealing speed. The heat transfer can be improved by guiding the ribbon through a narrow channel of an annealing fixture, which acts as the heat reservoir. However, for a reasonably wide opening, the ribbon tends to move freely through the channel and contacts the walls of the annealing fixture more or less accidentally, which results in a badly defined thermal contact and, thus, in a limited annealing speed.
It is an object of the invention to provide a method and apparatus for annealing a continuous ribbon of material with improved processing efficiency.
It is a further object of the invention to provide a method and apparatus for annealing a ferromagnetic, metallic ribbon in order to achieve characteristic magnetic properties at higher annealing speeds than achievable by conventional methods taught by the prior art without degradation of said properties.
It is another object of the invention to provide a method and apparatus which reduces an initial, e.g. production inherent, curvature of the ferromagnetic metallic ribbon with the proviso that this curvature-reduction is relatively insensitive to the precise annealing conditions (e.g. time and temperature) over a wide range and that it does not degrade other physical properties of the ribbon.
The above objectives can be accomplished by transporting the ribbon lengthwise on a path through a channel in a heat treatment fixture, in which along at least part of the channel protrusions extending transversely of the path cause the ribbon to wriggle and make multiple contacts with the heat treatment fixture, thereby making improved thermal contact with the heat treatment fixture. The objectives can also be accomplished by passing the ribbon lengthwise on a path through a channel in a heat treatment fixture, in which the path curves along a curved section of the channel causing the ribbon to make contact with the heat treatment fixture, thereby making improved thermal contact with the heat treatment fixture.
The protrusions and curved sections may be provided by undulations in the channel walls, which may be up and down curvatures along portions of its length. Along the curved portions of the channel the ribbon is forced into well-defined close contact with the walls of the channel, which significantly improves the heat transfer into the ribbon as compared to straight channels of the prior art. As a consequence the material is heated up much quicker to the temperature of the oven, which allows one to increase the annealing speed and/or build shorter annealing ovens.
Preferably the curved portion of the channel is located at the beginning of the annealing fixture, i.e. where the ribbon enters into the oven. Once sufficient heat has been transferred into the ribbon, the channel can be given a straight form again. The channel then acts as heat reservoir, which holds the ribbon at the annealing temperature.
It may be necessary that the annealing temperature reveals a certain profile, i.e. that the temperature changes along the length of the oven. Accordingly it may be advantageous that the annealing channel reveals curved sections at the locations where the oven temperature changes.
When the ribbon exits the oven it is still hot, which is a problem in particular for high annealing speeds. In another aspect of the invention, the annealing fixture therefore extends beyond the oven and contains a cooled portion, which again reveals a curved section. This guarantees a quick cooling of the ribbon, which may also be critical for the achievable properties.
When the hot ribbon is guided over a curved section, this curvature is annealed into the ribbon at least in part. Thus if the annealing fixture were curved over its whole length, the annealed ribbon would reveal an according curvature. In order to keep the annealed strip flat it is therefore preferable that the annealing fixture is essentially straight and that an xe2x80x9cup curvaturexe2x80x9d is followed by a xe2x80x9cdown curvaturexe2x80x9d or vice versa. Similarly the ribbon is also kept straight when a single up or down curvature of the channel is followed by a non-curved portion of at least the same length as the curved portion.